The rate of performance of disk storage devices has not kept pace with the improvements made in other aspects of data processing systems. In the more recent past, while the access times have decreased by a factor of 2 or 3 and data rates have improved by a factor of 2, there presents an ever widening gap between the performance of data processing units and the disk storage system. To a large extent, this is due to the existence of physical laws which require increasing amounts of energy to move mechanical components at higher velocities.
Overall storage system performance can be characterized by the amount of time required to retrieve a typical amount of data, say 16 kilobytes (KB). In the last five years, this time has decreased by a factor of 3. In the same period, the performance of solid state components of the system has improved by a factor of 10. This disparity shows no signs of improving, and more likely, will continue to worsen.
Looking at performance in terms of a midrange data processing system, which is typically used to service a large number of terminals, approximately 60% of the time is consumed by the storage subsystem. Typically, each user transaction requires about 20 disk operations, each of which transfer about 4 KB of data. This type of system provides a response time to the terminal operator of 1 to 2 seconds, which, while it is the actual value typically encountered, is not considered to be ideal by the typical terminal operator.
It can be seen that the response time is largely limited by the performance of the storage subsystem, and this will be even more the case as the performance gap between solid state devices and disk storage systems continues to widen. Adding to the severity of the problem is the fact that future applications are likely to be more complex and require more data to be transferred. The number of disk operations per user transaction is projected to rise from 20 to 40 and the average data transfer rate will increase from 4 KB to 20 KB.
The problem is further complicated by the fact that data processing systems are becoming a more essential and integral part of every day business and personal life. As such, it is no longer convenient, or even possible, for a user to wait for a skilled service representative to make a repair within a day or so. In most cases, the system will have to be restored to service within a matter of minutes, or at best, an hour or two, or valuable business will be lost. This means that the trouble shooting and repair or replacement of the defective unit will be done by the user himself.
Various approaches to the design problems of latency, size, power consumption, data rate, bit density, track density, serviceability and reliability of disk storage devices have been employed in the past. U.S. Pat. No. 3,876,978 assigned to the assignee of this invention describes a storage system having multiple units. One of the units is utilized to store a parity bit which is based on the data in the corresponding bit positions for each of the respective units. As described, the system relates to a tape cartridge system in which one cartridge is used to represent the parity bit resulting from a comparison of the corresponding bit in all the other cartridges. If a single cartridge is lost, the data can be regenerated by reading all the bits from all the data cartridges, combining and comparing the result of the corresponding bit in the parity cartridge and regenerating the data to produce the proper parity. The approach used in this patent, with but a single access mechanism, is well suited to use in a multiple disk drive system. In a disk drive system it would be desirable to perform an on-line check of the validity of all data read from the system as well as regenerate lost data. The use of the system described in the patent would not provide satisfactory performance for on-line checking of data read from the disk system.
The system described in U.S. Pat. No. 4,036,659 is representative of programmed controllers for disk drive systems. The very lengthy and highly detailed description for distributing data among a plurality of disk drives does not suggest the use of exceptionally small disks operating on very high rotational speed.
U.S. Pat. No. 4,568,988 describes a disk storage system utilizing disks in the range of 85-100 mm (96 mm) in diameter. The system uses an open loop track accessing system and operates at the conventional 3,600 RPM. The patent does not suggest that any advantage could be obtained by combining a plurality of such systems and distributing the data over all the drives in the system. Further, the suggested range of disk diameter does not extend below 85 mm.
video recording system described in U.S. Pat. No. 4,577,240 relates to a disk storage system which uses separate actuators to record the problems of error correction or recovery from a disk failure. The tracks which contain flaws which would otherwise produce errors are simply skipped. In actual data processing practice, this would lead to an intolerable loss of data capacity since single bit errors would result in the loss of an entire track. This is an unacceptable trade-off when applied to the problem of data storage. The patent also fails to suggest the use of more than one drive spindle, using separate actuators instead. This approach does not lend itself to the simple replacement of a failing unit since two actuators, and therefore also two groups of data, would be involved.
The use of multiple spindles in a disk storage system is suggested by U.S. Pat. No. 4,583,133. However, in this system only one drive is in use at a time. There is no teaching that the data could be simultaneously apportioned and recorded among all the units. The use of flexible media, rather than a hard disk, is contemplated, and there is no mention that the rotational speed is other than the low speed which is conventional for flexible media.
The teaching of U.S. Pat. No. 4,724,495 is directed to a video recording system using two separate actuators for recording successive video fields on different zones of the disk storage stack. There is no suggestion that separate spindles be used and the data simultaneously recorded on more than one spindle. This system handles defects in a track by simply skipping the entire track, similar to the approach shown in U.S. Pat. No. 4,577,240 discussed above. The spindle speed is specified as either 3,000 or 3,600 RPM. The patent does not suggest that a plurality of small disks be combined in a system in which the data is distributed across all the disks in parallel fashion.
Despite the attractive characteristics of such devices, there are some inherent limitations which have not been overcome. For example, the fact that data is arranged serially along a circular track makes it necessary to wait until the desired data passes under the data transducer. From a simple statistical standpoint, the average time required for the desired data to reach a transducer will be the period of time for one-half revolution of the disk. It is possible to modestly improve the simple statistical average latency by skewing the sectors from one track to another. This may allow a transducer to begin reading a track somewhat sooner after a track access than would otherwise be the case. The waiting time for the data to come under a transducer is referred to as latency.
While the time required for mechanical movement of the head-arm assembly during a track-to-track access has been substantially reduced with faster and faster actuators, there has been little improvement in the latency. This is easily understood when disk drive specifications are examined. The so-called "hard" drives, that is, those which utilize a rigid substrate, invariably rotate at 3,600 revolutions per minute. Since latency is inversely and directly related to rotational speed, it is not possible to make substantial improvements unless the rotational speed is increased.
It would seem to be a simple matter to speed up the spindle drive motor, and thereby reduce latency; however, such is not the case, as evidenced by the absence of disk drives of higher rotational speed. Some of the problems are obvious, even though they do not lend themselves to obvious solutions. For example, the so-called "Winchester" technology which provided for aerodynamic control of the head to maintain the head-disk spacing in the region of 8 to 25 microinches, relies on the existence of a thin film of air moving at a velocity of 600-1,000 inches per second. With disks rotating at 3,600 RPM and having a diameter of 3.5 to 8 inches or greater, this velocity requirement is easily satisfied and "sliders", which are the transducers arranged in an aerodynamic package, can be designed to operate satisfactorily for these parameters.
It has been long recognized that latency is directly related to rotational speed, and there is a direct benefit in the reduction of latency as a result of increasing rotational speed. Nevertheless, the rotation speed of virtually all hard disk drive systems has remained in the 3,000 to 3,600 RPM range.
This is at least partially explained by the fact that, for example, tripling the speed to approximately 10,000 RPM, to obtain a worthwhile improvement in latency, represents a drastic departure from existing slider technology. Such a departure could require a costly redesign of the slider to accommodate the altered aerodynamic situation resulting from the increased disk velocity.
The aerodynamic problem is predictable and presumably could be solved with sufficient engineering effort. Such is not the case with other problems. It has been recognized that the occasional contact between the head and the disk can lead to premature failure of the device if appropriate precautions are not taken. It is possible to design mechanisms which position the transducer over the "landing" zone, where no data is stored, when the disk is started and stopped. This eliminates a portion of the wear and potential damage to the disk surface over which data is stored. There is still the potential for inadvertent contact between the transducer and the disk during normal reading and writing of data. Thus, even where economic justification exists for the cost of special mechanisms to position the transducer of the landing zone, it is desirable to have some form of protection on the disk surface. Typically this protection takes the form of a lubricant which is applied to the surface of the recording media on the disk.
The development of suitable lubricants has been difficult. The requisite lubrication characteristics eliminate all but a few classes of lubricants. In addition, the lubricant must not interact with contaminants to form physical structures which would interfere with the contaminants found in the usual business or home environment to form molecules, crystalline or amorphous structures which approach a substantial portion of the normal head-disk spacing, it is possible that the head would come into contact with the structure leading to a "head crash" and resulting damage to the disk surface and loss of data. Not only must the lubricant be satisfactory from the lubrication and chemical standpoint, it must also possess certain physical characteristics which cause it to adhere to the disk surface and not be spun off. It has been found that some lubricants which possess the desired chemical and lubrication characteristics are prone to migration to the periphery of the disk as a result of the centrifugal force. Since this failure mode appears only after long periods of operation, it is difficult to evaluate and test lubricants for satisfactory performance. Lubricants which operate to produce satisfactory disk life at rotational speed of 3,600 RPM would require, at a minimum, extensive life testing to verify their acceptability at higher rotational speeds. The likely result of such testing would indicate that existing lubricants are not satisfactory for use at substantially higher speeds.
It is therefore understandable that rotational speed have remained fixed at 3,600 RPM since the foreseeable problems and unpredictable results which would arise from higher speeds constitute a substantial deterrent to the investment in development.
If higher speeds were to be attempted it would be anticipated that the heat dissipated within the drive would lead to problems. The increased amount of power needed to rotate the disk at higher velocity would necessarily lead to a larger spindle drive motor with its attendant greater power dissipation. This is not a trivial problem when it is recognized that the increased power dissipation would be within a very small volume, making dissipation of the heat even more difficult. It could also be anticipated that there would be increased windage losses and this would also contribute to the heat developed within the device.
Since space is always a consideration, the larger drive motor would be a substantial deterrent to the use of higher rotational speeds. This is particularly the case in the so-called personal computers, in which the disk drive already occupies an inordinate fraction of the available space and power. With the trend toward increasing small personal computers, a disk drive which would occupy more space than existing disk drives would not be acceptable. Additional space would very likely be required for the heat sinking of the higher speed unit.
Even if it were possible to increase rotational speed for the purpose of improving latency, such a development would inevitably lead to an effort to improve the actuator access time and this would also tend to increase the power dissipated within the device.
The prior art bears ample evidence of the contradiction which exists between the efforts to improve latency and the other requirements for disk drives. The greater power and size of the device which would have improved latency contradicts the requirement that devices be made ever smaller, consume less power and run cooler. On top of all this are the inherent requirements that the device be more reliable and be less expensive.
Another problem which has not been successfully addressed in the prior art relates to the repair and/or replacement of defective disk drives. This task has traditionally required a high degree of skill, well beyond that of the average personal computer user. The repair of hard disk drives is even more critical than the less sophisticated flexible disk drive because of the nature of the data stored on the respective types. Typically the hard disk will contain all the application programs used on the system and it may also contain the bulk of the data. Because of the critical nature of the information on the hard drive, it is customary to periodically perform a "back up" operation, which is essentially a duplication of the data on the hard disk on a flexible disk or tape type device. The flexible disk or tape can be used to recover the data if the hard disk should be damaged or otherwise become inoperable.
While the disk drives are designed to perform error correction on erroneous data, the power of such error correction capability is customarily severely limited and cannot handle the loss of large blocks of data which result from a head crash of even modest proportions.
The back up operation is a nuisance to perform and the requirement for a back up is frequently ignored until after the hard drive fails, when it is too late to do anything about recovering the data. The replacement of a disk drive is often a complex task, involving considerable expertise in both the mechanical aspects of removal and replacement of the drive, as well as software knowledge to format the disks and reload such information as may still be available.
An additional problem with the prior art disk storage systems relates to the material used for the disk substrate. Aluminum has been the material of choice almost from the outset of the technology. Aluminum offers the advantage of light weight, good machineability and strength. At previous values of magnetic media thickness and head flying heights, the aluminum substrate could be turned to an adequate finish with conventional, albeit it expensive, machining techniques. However, even with the most advanced machining equipment, the surface finish on an aluminum substrate includes imperfections which are beyond the tolerance for the thin media coatings necessary to improve bit and track density.
The surface finish must be quite perfect to provide an error free disk. After a disk has been coated with magnetic media, the surface can be finished to a very fine surface. While this reduces the severity of the problem with head crashes, the existence of minute projections or pits in the substrate will result in bit drop outs in the region of projections and bit spreading in the region of pits even though the surface of the media is virtually perfect. The most advanced techniques for finishing the surface of aluminum substrates approach the theoretical limits of the material. The use of pure aluminum provides the best potential for a good finish since it does not contain the impurities and inclusions common to aluminum alloys. Unfortunately, pure aluminum is quite soft and the machineability is so poor that it limits the quality of the surface that can be obtained. Other materials have been evaluated for use as substrates. Both glass and semiconductor grade silicon have been used. Either material provides the potential of a much better surface finish than can be obtained with aluminum. Ceramic substrates may also be used. However, these alternatives are more brittle than aluminum. This characteristic has prevented their widespread use, particularly in the type of drive which is used in the personal computers, which must be relatively more rugged to stand the abuse of relocation by unskilled persons. Additionally, the mechanical strength of ceramics, glass and semiconductor silicon, which is adequate to withstand shock and the stresses of operation at the present rotational speeds, become suspect when the speed is increased to much higher values.
While the performance of the actuators which position the transducer over the desired track has been continuously improving to provide ever faster access times, the improved performance has come largely at the expense of greater expenditure of power. This produces complications for the overall computer system in terms of a higher capacity power supply, reduced operating time for battery operated systems, and additional heat produced within the drive.
It has been discovered that the performance of large systems using high performance disk drives reaches a constraint imposed by the vast amount of data which is accessible by each actuator. Because access to the data on each such drive necessarily proceeds in serial fashion, there is an unavoidable delay associated with the retrieval of data. While this limitation could be avoided by placing less data under each actuator, the high cost of each actuator and the associated components of the disk drive system have made such an approach economically impractical. In other words, it does not make sense to utilize a high cost actuator unless it is combined with a large amount of data. The goal of a high speed actuator to access relatively smaller amounts of data has not been realizable in a commercial environment.
Further, while the advantages of having less data under each actuator have been recognized, past approaches to this problem have suffered from the loss of reliability due to the existence of additional actuators.
In summary, it would be desirable to have a disk drive system which provided improved latency, but which is smaller and uses less power than existing devices. The device would ideally utilize as much of the existing disk drive technology as possible to allow use of existing lubricants and slider technology. These improvements would ideally be accompanied with improved actuator performance to accompany the reduced latency. All of this should be accomplished without reduced reliability. Preferably the system should allow the customer to replace defective portions of the disk system without the use of complex tools, special mechanical skills or programming skills beyond those possessed by an average user.